Aircraft braking method and apparatus

ABSTRACT

An aircraft is decelerated by applying a braking force to a wheel of the aircraft as it moves along the ground. An anti skid controller calculates the braking force to be applied by taking into account data relating to the vertical load transmitted between the ground and the wheel and data relating to the slip between the ground and the wheel. Predictions (box  13   b ) made regarding how the vertical load will change and data (box  13   a ) concerning the relationship between slip and the ground to wheel friction coefficient are both taken into account when calculating the braking force to be applied. Skids may thereby be predicted in advance and may be reduced or even avoided.

BACKGROUND OF THE INVENTION

The present invention relates to decelerating an aircraft and, inparticular, to a method of applying a braking force to the wheel of anaircraft and to a braking apparatus for performing such a method.

After having touched-down on landing, an aircraft is caused todecelerate in various ways, one example of which being by means ofapplication of a braking torque to the wheels of the aircraft. It isdesirable to decelerate the aircraft in an efficient and controlledmanner. Thus, it is desirable to maximise the pilots demand fordeceleration of the aircraft by means of the braking torque applied tothe wheels. The wheels of the aircraft are prone, when subjected to asufficient braking torque, to skidding and may, in severe cases, lock-upcompletely. It will be appreciated that there is generally some slipbetween the wheels and the ground when the aircraft is moving, but thatabove a given amount of slip the wheels can be considered as skidding.When the wheels are skidding the ability of the aircraft to decelerateby application of the brakes is impaired. Aircraft are thereforecommonly provided with anti-skid systems.

A known aircraft anti-skid system for a single wheel of an aircraftmonitors various parameters including, in particular, the rotationalspeed of the wheel and the speed of the aircraft. From the measuredvalues of the rotational speed of the wheels and the speed of theaircraft, and from knowing the rolling radius of the wheel, the amountof slip, λ, may be calculated from the equation:λ=1−ωR/V, where

-   ω=the rotational speed of the wheel,-   R=the rolling radius of the wheel, and-   V=the speed of the aircraft.

If the slip λ increases above a given threshold indicating that theaircraft has started skidding, the braking torque is reduced until theslip has decreased below the threshold. The known system suffers fromcertain disadvantages. In particular the system tends to be reactive asopposed to being proactive. For example, the system allows the wheels toskid before reducing the braking torque.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of brakingand a braking apparatus that mitigates one or more of theabove-mentioned disadvantages and/or to provide an improved method ofbraking and an improved braking apparatus.

The present invention provides a method of applying a braking force to awheel of an aircraft moving along the ground, wherein the methodcomprises the steps of performing a calculation taking into account aparameter

relating to the vertical load transmitted between the ground and thewheel, and

applying a braking force to the wheel in dependence on the results ofthe calculation so performed.

By taking into account a parameter dependent on the vertical loadtransmitted between the ground and the wheel the braking force may beapplied more efficiently and effectively. For example, by means of themethod it may be possible to anticipate a skid, before the skid starts,and react accordingly. Thus, use of the method may be able to improvesignificantly the braking capability of the brakes of an aircraft andmay be able to reduce the amount of time during which the wheels wouldotherwise have been skidding. Also, if and when a skid does occur, anyinformation concerning the vertical loads at the time of the skid mayadvantageously be used in the future to help prevent, or at least helpcontrol, further skids. Said step of performing a calculation may, butneed not necessarily, form part of a step of estimating the conditionsat which the wheel would skid. In that case, the application of thebraking force to the wheel is preferably applied in dependence on theresults of the estimating step.

The method may be so performed that a change in the vertical loadtransmitted between the ground and the aircraft results in asubstantially proportional change to the parameter relating to thevertical load transmitted between the ground and the aircraft. Theparameter may for example be substantially equal to the vertical load,or could be such that the parameter is equal to the vertical load oncescaled and possibly off-set.

The method may alternatively be performed so that a change in the timederivative of the vertical load transmitted between the ground and theaircraft results in a change to the parameter relating to the verticalload that is substantially proportional to the change in the timederivative of the vertical load. The parameter may for example be ameasure of the rate of change of the vertical load. The method may, insuch cases, include a step of calculating the change in the verticalload over a given length of time, for example by a method ofintegration.

The parameter relating to the vertical load may be ascertained bymeasuring a physical quantity, for example, the load itself, orquantities that vary primarily in dependence on the vertical load. Theparameter may for example be measured by means of stress sensorspositioned, for example, in or on the landing gear.

The parameter relating to the vertical load may be ascertained byestimating the vertical load from other measured parameters relating toother aspects of the aircraft, its movement, or other variables. Forexample, the vertical load may be estimated by performing a calculationin which one or more of the following parameters are taken into account:landing gear oleo shock absorber pressure, the extension of one or morestrain gauges, for example, mounted on or in the landing gear of whichthe wheel is part, and acceleration of the aircraft. There may forexample be provided one or more strain gauges which are able to measurethe strain in relation to a single wheel, or in relation to a pair ofwheels or in relation to a group of wheels. There may be one or moreoleomatic sensors to measure the shock absorber pressure. There may beone or more accelerometers including for example monitoring devices thatmeasure one or more of pitch roll, yaw and translational acceleration inany of three substantially orthogonal axes.

The calculation advantageously also takes into account at least oneother parameter. Said other parameter advantageously providesinformation regarding the amount of slip. For example, the parameter maysimply indicate whether or not a skid has been entered. Preferably saidother parameter is at least partly dependent on the amount of slipbetween the wheel and the ground. For example, a slip parameter may betaken into account when performing the calculation, the slip parameterbeing such that the amount of slip between the ground and the wheel andthe slip parameter are interrelated. The slip parameter may be directlyor indirectly related to the amount of slip. For example, the slipparameter may relate to the horizontal loads in the direction of travelof the aircraft sustained by an axle of the wheel. The braking forceapplied may be controlled in dependence on both the parameter relatingto vertical force and the slip parameter. The changes in eitherparameter from previous values may also be taken into account whencalculating the braking force to be applied. Said other parameter mayadditionally or alternatively relate to the amount of braking torqueapplied to the wheel.

Data is advantageously ascertained regarding the relationship betweenslip and the ground to wheel friction coefficient. At least some of thedata so ascertained is preferably used in the calculation, which affectsthe amount of braking force applied.

The method may include recording, over time, data relating to therelationship between the value of the friction coefficient and the valueof slip. Having information regarding the slip/friction coefficientcurve can assist in predicting the value of slip where a region ofstable braking ends and the value of slip where a region of unstablebraking begins. Such information is advantageously used to maximise thebraking force applied, whilst seeking to avoid conditions at whichskidding starts. Thus, the method preferably uses the data so recordedto improve the efficiency and effectiveness of braking. Preferably, theefficiency of braking is improved by means of a control unit thatincreases the braking such that the level of slip nears, but does notexceed, a level at which unstable braking starts, the control unit usingthe data recorded in order to assess the point at which unstable brakingstarts. The point at which unstable braking starts may be considered asthe value of slip at which the friction coefficient is a maximum. Thecontrol unit may of course monitor one or both of the values relating tothe slip and the friction coefficient to assess the point at whichunstable braking begins.

The method preferably includes a step in which the slip parameter isascertained by measuring a physical quantity. The physical quantitymeasured may for example be a measure of the horizontal load sustainedby the aircraft or by a part of the aircraft.

The slip parameter relating to the slip between the ground and the wheelmay be ascertained by means of measuring parameters relating to theaircraft speed and the speed of the periphery of the wheel. The slip, λ,may then be calculated by means of a formula identical or equivalent to:λ=a _(o) −a ₁ ωR/V, where

-   ω=a parameter relating to the rotational speed of the wheel,-   R=a parameter relating the rolling radius of the wheel,-   V=a parameter relating the speed of the aircraft,-   a₀=a constant that is preferably 1, but may be zero or any other    value, and-   a₁=a constant that is preferably 1, but may be minus one, or any    other value.

The parameter ω may be the angular velocity of the wheel (for example,in radians per second i.e. equal to 2 Pi×Revs/second). The measuredparameter may of course be equal to the number of revolutions per unittime, t (for example, the inverse of the time per revolution, which canbe measured in a conventional manner). Ascertaining the parameter co mayfor example comprise a step of using means that are conventionally usedto provide an indication of the rotational speed of the wheel.

As indicated above, formulae equivalent to λ=a_(o)−a₁ωR/V may be used.For example, formulae such as λ=a_(o)V−a₁ω, or λ=a₀V/ωR−a₁ could be usedto give a measure of the relative velocity between the periphery of thewheel and the ground.

The method may include a step in which a prediction is made regardinghow the vertical load will change so that the braking force may bechanged accordingly. That prediction may then be used to predict themaximum level of braking that can be applied whilst taking into accountthe desire to minimize the risk of causing the wheels to enter into askid. Preferably, the step includes changing the braking force.Preferably, the method includes a step in which a prediction is maderegarding how the vertical load will change and the prediction is takeninto account when performing the calculation and/or the estimating step.

For example, it may be ascertained that the vertical load is increasingand a prediction may be made as to what the increase will be within anotional period of time. An increase in vertical load will generallyincrease traction and therefore reduce the slip encountered, whereas adecrease in vertical load will generally decrease traction and thereforeincrease the slip encountered. Therefore, on a predicted increase in thevertical load the braking force applied will advantageously beincreased, because traction will have increased so that the chance of askid is reduced, whereas on a predicted decrease in the vertical loadthe braking force applied will advantageously be decreased, to reducethe chance of encountering a skid that might otherwise have been caused.When an aircraft lands the vertical loads sustained at a given wheeltend to oscillate over time. The sign of change (i.e. positive ornegative) of the vertical load at a given time can therefore bepredicted with reasonable accuracy. The magnitude of the change invertical load in a given time is preferably also estimated.

The method advantageously includes a step of estimating the conditionsat which the wheel would skid. The method of performing a calculationmay for example include such an estimating step. The estimating steppreferably takes into account the vertical load transmitted between theground and the wheel (or the aircraft). The step of applying a brakingforce to the wheel is preferably performed such that the braking forceapplied depends on the results of the estimating step. Preferably thebraking force applied is adjusted to a level at which it is judged thatthe conditions for skidding will not be met whilst maintaining effectivebraking.

The method is preferably so performed that, if a skid is detected, thebraking force is reduced in a way that takes into account data relatingto the vertical load transmitted between the ground and the wheel.Controlling the reduction and subsequent increase of the braking forceapplied in view of information relating to the vertical load and/or inrelation to the slip/friction coefficient curve is particularlyadvantageous. For example, an estimate of the profile of the brakingforce over time that should be applied to maximise the braking of thewheel over that time can be improved with the use of such information.

The method may include a step of ascertaining a parameter relating tothe vertical load transmitted between the ground and the aircraft andoutputting a first value dependent on that parameter. The calculationperformed may involve the use of said first value. The method may alsoinclude ascertaining and outputting a second value representative of afurther parameter, for example the amount of slip between the ground andthe wheel. The calculation performed may involve the use of both theafore-mentioned first and second values.

The method preferably includes a step of ascertaining a parameterrelating to the braking force applied to the wheel. The parameter ispreferably taken into account when calculating the braking force to beapplied to the wheel. The brakes may be actuated by means of a hydraulicsystem. In such a case, the method may for example include a step ofascertaining a parameter representative of the hydraulic pressure in thebrake system, the parameter being taken into account when calculatingthe braking force to be applied to the wheel. The ascertaining of theparameter may comprise measuring the hydraulic pressure in brake system.The method may include ascertaining or estimating the braking torque orforce applied to the wheels by means of a calculation involving aparameter relating to the braking pressure.

The method may include a step of estimating how the braking torqueapplied to the wheel changes with changes in other variables and varyingthe brake pressure applied to account for the changes in such othervariables. For example, such other variables may include brake discrelative rotational speed, brake temperature, and moisture content inthe brakes. The braking torque is likely to change over time, when aconstant braking pressure is applied. Wheel speed, brake temperature andpossibly other variables each affect the amount of braking torqueresulting from the application of a given braking pressure. For example,as the amount of moisture in the brakes is reduced, the braking torqueresulting from the same brake pressure may increase. By monitoring andassessing, how the braking torque varies with such variables, thebraking pressure applied can be varied over time, in view ofmeasurements made relating to those variables, in such a way as toaccount for the changes in the braking torque that are dependent onthose variables. Assessing how the braking torque varies with changes insuch variables can of course be assessed even when other parametersincluding the brake pressure are also varying. The braking pressure maybe varied by taking into account the time that has elapsed since thebrakes were actuated. Look-up tables may be stored electronically toenable a first approximation to be made regarding the braking torquethat is being applied in relation to a given braking pressure, and inview of one or more other variables. Such look-up tables can be adjustedto take account of measurements made under the prevailing conditions,thereby improving the accuracy of the look-up tables.

Preferably, the parameter relating to the braking force is ascertainedfirstly by making an estimate of the braking force/torque, thenestimating, by taking other known or measured parameters into account,how one or more such parameters, for example the angular velocity of thewheel, might change in a given period of time, then measuring and/orcalculating said one or more such parameters and comparing themeasured/calculated value(s) with the estimated values, and thenimproving the estimate of the braking force/torque in view of thecomparison made. The estimate of the braking force may be improved byestimating the friction coefficient and using the resulting value toimprove the estimate of the braking force. Such a method of ascertainingthe parameter relating to the braking force/torque may be performed manytimes in a period of 0.5 seconds. Preferably the method is performed insuch a way that the process is iterative in nature, for example, withthe aim that the estimates become progressively more accurate. Areasonably accurate estimate of the braking force may thus be made.

The braking force or braking torque may alternatively be measureddirectly, with for example torque sensors.

Preferably the method is performed in such a way that the pilot isunable to have control of the braking of the wheels before the wheelshave spun-up to a pre-set minimum rotational speed on touch-down. Thebraking of the wheels may however be under the overall control of thepilot after initial spin-up of the wheels. The level of maximum brakingmay be under the control of the pilot for at least some of the timeduring landing, the method of the invention providing automatic controlif the level of braking needs to be reduced (or increased) at a giveinstant. The method may be performed in such a way that braking of thewheels is completely automated up to a point in time at which the speedof the aircraft decreases to below a pre-selected speed, for example,taxiing speed.

The invention also provides a method of decelerating an aircraft inwhich the above-described method is performed in respect of amultiplicity of the wheels and preferably in respect of the majority of,and preferably all of, the wheels to which a braking force is applied.The method is of particular advantage during the landing of an aircraft.

According to a further aspect of the invention there is provided amethod of applying a braking force to a wheel of an aircraft movingalong the ground, the method comprising the steps of a) estimating theconditions at which the wheel would skid, the estimating step takinginto account the vertical load transmitted between the ground and thewheel, and b) applying a braking force to the wheel in dependence on theresults of the estimating step.

The invention also provides a braking control apparatus, and a processorassociated with the braking control apparatus, for controlling thebraking of an aircraft wheel in accordance with the method of thepresent invention. According to an aspect of the invention there is thusprovided a braking control apparatus for controlling the braking of anaircraft wheel and a processor associated with the braking controlapparatus, the apparatus being connectable to the brakes of at least onewheel of an aircraft and the processor being able to be connected toreceive in use signals relating to the vertical load transmitted betweenthe ground and the aircraft wheels, the processor being so arranged thatin use it performs a calculation using data derived from the signalsreceived by the processor, wherein the control apparatus is so arrangedthat in use the control apparatus actuates the brakes in dependence onthe results of the calculation performed by the processor, whereby thecontrol apparatus is able to control the actuation of the brakes takinginto account the vertical load. The present invention also provides,according to a further aspect of the invention, a braking controlapparatus for controlling the braking of an aircraft wheel and aprocessor associated with the braking control apparatus, the apparatusbeing connectable to the brakes of at least one wheel of an aircraft andthe processor being able to be connected to receive in use signalsrelating to the vertical load transmitted between the ground and theaircraft wheels, the processor being so arranged that in use it performsa calculation using data derived from the signals received by thecontrol apparatus and estimates the conditions at which the wheel wouldskid, the estimating step taking into account the vertical loadtransmitted between the ground and the wheel, wherein the controlapparatus is so arranged that in use the control apparatus actuates thebrakes in dependence on the results of the calculation performed by theprocessor, whereby the control apparatus is able to control theactuation of the brakes taking into account the vertical load and otherconditions that affect the likelihood of skidding.

Optional and/or preferred features relating to either aspect of theinvention described above relating to the control apparatus and/or theprocessor will now be described.

The control apparatus and/or the processor may be supplied separatelyfrom the aircraft to allow the present invention to be retrofitted. Anexisting control apparatus and/or processor may be able to be convertedand reprogrammed to a control apparatus and/or processor according tothis aspect of the present invention. Of course, in use, the controlapparatus will be connected to the brakes of an aircraft and theprocessor will be connected to one or more sensors. Such connections maybe direct or indirect. For example, other control systems or processorsof the aircraft may be required to ascertain data relating to variousparameters and the processor of the present invention may simply usethat data. The processor may form a part of the control apparatus. Theprocessor is advantageously able to receive, in use, signals relating tothe amount of slip between the ground and the wheel so that data derivedfrom such signals may also be used in the calculations performed by theprocessor. The processor is preferably connectable in use to receivesignals from a wheel speed sensor and an aircraft speed sensor. Theprocessor may be connectable to one or more accelerometers. Theprocessor may be connectable in use to a strain gauge or a force orpressure sensor in order to assess the vertical load transmitted betweenthe ground and the aircraft. The processor may for example receivesignals from an air data inertial reference unit (commonly referred toby the acronym ADIRU).

Preferably, the control apparatus and processor are so arranged that theapparatus is able to perform the method according to the presentinvention. The processor is advantageously programmed to perform one ormore of the various aspects of the above-described method. For example,the processor may be programmed to estimate how the vertical load willchange in a given period of time. The processor may then estimate thebraking force that, if applied, would result in a given amount of slip(preferably the optimum slip for efficient and effective braking) andoutput a signal that causes the brakes to be actuated in accordance withthe braking force so estimated. The processor may also estimate theconditions at which the wheel might skid by taking into account datarelating to the vertical load transmitted between the ground and theaircraft and data relating to the slip between the ground and the wheel.The processor may then send an output signal which causes the brakes tobe actuated to provide a braking force at such a level that theconditions for skidding are not met whilst maintaining effectivebraking. The processor advantageously monitors the slip and possiblyother parameters for the start of a skid. The processor is preferablyarranged so that if a start of skid is detected the braking force isreduced, preferably in a way that takes into account data relating tothe vertical load transmitted between the ground and wheel. Theprocessor advantageously calculates the slip by means of a calculationusing data regarding the aircraft speed and the speed of the peripheryof the wheel. The data regarding the speed of the periphery of the wheelmay simply be data representative of the revolutions per unit time ofthe wheel, which together with the knowledge of the rolling radius ofthe wheel can be used to calculate the peripheral speed of the wheel.

The processor may be provided with a memory in which data may be storedto improve the performance of the control apparatus. Data may forexample be ascertained regarding the relationship between slip and theground to wheel friction coefficient. Such data may be stored in thememory. Each time a skid is encountered, the processor preferably storesin the memory data regarding the relationship. Such data may for examplebe able to be used later to provide valuable information regarding theconditions at the time of landing (for example information regarding thecondition of the runway). The processor may for example use such datawhen calculating the braking force to be applied to the wheel.

The braking force may be chosen on the basis of an estimate of the forcewhich would achieve the maximum ground to wheel friction coefficient.The estimate is preferably calculated in view of the data regarding therelationship between the friction coefficient and slip stored in memory.The anticipated effect on the relationship between slip and the frictioncoefficient of a change in other parameters, including for example thevertical ground to wheel load, may also be taken into account. Theprocessor preferably stores in memory several sets of data relating tothe relationship between the friction coefficient and slip. Each time askid is started a new or additional set of data concerning therelationship may be stored in memory.

The memory may be used to store other data. For example, data may bestored regarding the signal level required to be sent to the brakes inorder to achieve a desired braking torque or braking force at the wheel.For example, if the brakes are actuated by means of hydraulic system,the memory may store data concerning the hydraulic pressure required inthe brake system in order achieve a given brake force at the wheel. Asis mentioned above, the brake torque applied in response to a givenhydraulic brake pressure is dependent on various factors. The memory mayinclude data concerning how the relationship between the brake torqueand brake pressure varies in response to changes in a further parameter.The processor may update and enhance this data by taking into accountmeasurements made during use of the control apparatus in order toimprove the accuracy of the data (or at least to improve the accuracy ofthe data in relation to the conditions at the time of use of the controlapparatus).

Also, the method of the present invention is preferably performed withthe use of a control apparatus and processor according to the presentinvention.

The present invention also provides a landing gear assembly connected toa control apparatus as described above.

The invention further provides a control unit and a landing gearassembly for an aircraft, the assembly including at least one aircraftwheel, the control unit being able in use to actuate the brakes of saidat least one wheel in accordance with the method of the presentinvention. According to an aspect of the present invention, there isthus provided a control unit and a landing gear assembly for anaircraft, the assembly including at least one aircraft wheel, thecontrol unit being able in use to actuate the brakes of said at leastone wheel, the control unit including a processor, which is connected toreceive data signals relating to the vertical load transmitted betweenthe ground and the aircraft wheels, and which in use performs acalculation using data derived from the data signals received by theprocessor, wherein the control unit is so arranged that in use thecontrol unit actuates the brakes in dependence on the results of thecalculation performed by the processor. According to a yet furtheraspect of the invention there is also provided a control unit and alanding gear assembly for an aircraft, the assembly including at leastone aircraft wheel, the control unit being able in use to actuate thebrakes of said at least one wheel, the control unit including aprocessor, which is connected to receive data signals relating to thevertical load transmitted between the ground and the aircraft wheels,and which in use performs a calculation using data derived from the datasignals received by the processor and estimates the conditions at whichthe wheel would skid, the estimating step taking into account thevertical load transmitted between the ground and the wheel, wherein thecontrol unit is so arranged that in use the control unit actuates thebrakes in dependence on the results of the calculation performed by theprocessor. In relation to either aspect of the invention described aboverelating to the control unit and a landing gear assembly for anaircraft, it will be appreciated that the processor may be located in adifferent physical location to the rest of the control unit.

There is further provided an aircraft including a processor, a controlunit and/or a landing gear assembly according to the above-describedinvention.

There is also provided a method of landing an aircraft includingperforming the steps of the above-described method.

Reference is made herein to steps of calculating, measuring and/orascertaining parameters, variables and the like. It will of course beunderstood that in at least some embodiments of the invention such stepswill be performed in such a way that the resulting numerical value(s)attributed to the parameter(s), variable(s) or the like will differ fromthe actual value(s) present in the physical system being modelled. Suchdifferences may result from errors in measurements made or may resultfrom errors introduced by the particular model chosen to represent thephysical system. The possibility of such errors can, if necessary, betaken into account or can, if sufficiently small, simply be ignored whenputting the present invention into practice.

An embodiment of the invention will now be described by way of examplewith reference to the accompanying drawings of which:

DESCRIPTION OF THE FIGURES

FIG. 1 shows a braking control system according to the embodiment;

FIG. 2 shows the tyre/ground dynamics in relation to the wheel of anaircraft on the ground;

FIG. 3 shows the characteristics of a tyre/ground interface in terms ofthe relationship between the friction coefficient and the amount of slipbetween the tyre and ground; and

FIG. 4 shows a flow diagram illustrating the method of operation of thebraking control system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The embodiment of the present invention relates to a braking controlsystem for controlling the braking of a wheel of an aircraft landinggear assembly during landing of the aircraft.

FIG. 1 shows a schematic diagram illustrating the operation of thebraking control system of the present invention. The system comprises ananti-skid controller 1 which is connected to a wheel speed transducer 2,a vehicle velocity transducer 3, a brake actuation transducer 4 and avertical load transducer 5. The anti-skid controller 1 is also connectedto a brake 6, which is able to effect a braking action on the wheel 7.

Vertical loads between the ground and the wheel are ascertained by meansof strain gauges on the landing gear, an electronic signalrepresentative of the vertical load so ascertained being sent from thevertical load transducer 5 to the anti-skid controller 1. Strain gauges,such as piezo-electric strain gauges, may be used to measure individual,paired or grouped wheel loads. The vertical load (or change in verticalload) may be calculated in respect of a wheel, or a number of, wheels.Conventional transducers are used in the system and include an aircraftground speed transducer, a brake pressure transducer, and a wheel speedtransducer.

During landing, the brake 6 is instructed by the controller 1, after forexample receiving an instruction from the pilot, to exert a brakingforce on the wheel and thereby cause the wheel to decelerate or stop.The anti-skid controller 1 receives signals from the wheel speedtransducer 2, the vehicle velocity transducer 3, the brake actuationtransducer 4 and the vertical load transducer 5, the signals beingrepresentative of wheel speed, and vehicle velocity, the pressure in thehydraulic system that actuates the brake, and the vertical load betweenthe wheel and the ground, respectively. The signals are used to assistin the efficient and effective use of the brakes as is described infurther detail below. In particular, the anti-skid controller 1 is ableto control the braking force applied by the brake 6 to the wheel 7 inorder to reduce the amount of skidding.

In order to understand better how the anti-skid controller 1 is able tocontrol the brakes in a more efficient manner it is useful to considerthe tyre/ground dynamics and the relationship between slip λ and thefriction coefficient μ.

FIG. 2 shows the tyre/ground dynamics of a typical wheel, tyre and brakeassembly. The system illustrated by FIG. 2 may be summarised by thefollowing equations:

$\begin{matrix}{{{I\frac{\mathbb{d}\omega}{\mathbb{d}t}} = {{R\; F_{F}} - T_{B} - T_{R}}}{F_{F} = {\mu\; F_{Z}}}{{I\frac{\mathbb{d}\omega}{\mathbb{d}t}} = {{\mu\; F_{Z}R} - T_{B} - T_{R}}}} & (1)\end{matrix}$Where:

-   I=Rotational moment of inertia,-   dω/dt=Angular acceleration of wheel,-   μ=Tyre/ground friction coefficient,-   F_(Z)=Vertical load acting on wheel,-   F_(F)=Frictional force due to braking=μF_(z),-   T_(B)=Braking Torque,-   T_(R)=Torque due to rolling resistance,-   V=Aircraft speed,-   ω=Angular velocity of wheel, and-   R=Rolling radius of wheel.

A skid will occur when the wheel speed has decreased sufficiently belowthe aircraft speed (V). The percentage difference between the aircraftspeed and peripheral wheel speed (the angular velocity multiplied by therolling radius) is defined as the slip of the tyre (λ), such that

$\begin{matrix}{{\lambda = {1 - \frac{\omega\; R}{V}}},} & (2)\end{matrix}$where

-   ω=Angular velocity of wheel,-   R=Rolling radius of wheel, and-   V=Aircraft speed.

A skid will normally either be due to an increase in braking torqueT_(B), or a decrease in tyre friction force F_(F). A decrease in tyrefriction force F_(F), which is equal to the product μF_(z), may be dueto a decrease in μ and/or F_(Z). The anti-skid system of the presentembodiment measures/infers vertical loads, or changes in vertical load,and can from that information predict when skids might occur, forexample when there are reductions in the vertical load F_(z).

FIG. 3 shows the characteristics of a tyre/ground interface by means ofa μ/slip curve. When the slip λ is greater than λ_(μMAX) the value of λat the maximum value of μ—see region 8 on the graph), an increase inslip will cause a decrease in μ and therefore a further increase inslip. Unless the braking torque applied is released, the wheel willrapidly decelerate and eventually lock-up. When slip is greater thanλ_(μMAX) there is effectively a positive feedback loop, which causesslip to increase very quickly. The region 9 b of the μ-λ curvecorresponding to the condition λ>λ_(μMAX) may thus be considered as aregion of unstable braking, whereas the region 9 a corresponding to thecondition λ<λ_(μMAX) may be considered as a region of stable braking. Itmay therefore be defined that a skid occurs when the slip increasesabove λ_(μMAX). Different conditions will change λ_(μMAX), μ_(MAX), theshape and slope of the μ/slip curve and the relative positions of thestable and unstable regions 9 a, 9 b.

The optimum braking efficiency is achieved when the slip is maintainedat or just below λ_(μMAX), which corresponds to the maximum coefficientof friction. Therefore, when the pilot demands braking that wouldotherwise cause a skid, the anti-skid controller 1 seeks to maintain thebraking torque so that the slip between the wheel and ground ismaintained at a level as close to λ_(μMAX) without significantlyexceeding λ_(μMAX), so that optimum braking may be achieved. In order toachieve this aim, the controller 1 ascertains information concerning theconstantly changing shape of the μ-λ curve and other changing parametersthat cause changes in μ and λ and adapts the braking torque appliedaccordingly. The function of the anti-skid controller 1 is described infurther detail below.

The dynamics of the tyre/ground interface are extremely complex andinfluenced by many factors; such as vertical-load, vehicle and wheelspeeds, brake actuation and rate of change of brake actuation, groundconditions, tyre wear, and tyre temperature. Conventional aircraftanti-skid systems have based the control of the said dynamics on themeasurement of the aircraft speed, wheel speed and brake actuation only.

The system of the present embodiment is able to use informationconcerning the vertical loads in several different ways to improve wheelbraking efficiency and effectiveness.

The anti-skid system uses the vertical load information to determinemore accurately when a skid could occur and, also when skids do occur,the causal reasons behind such skids. Knowing the cause of a skid canassist both in controlling the skid efficiently and possibly in helpingto avoid encountering a skid of the same, or a similar, cause again.

The system is able to predict impending skids by monitoring verticalloads or changes in vertical loads. Predictions are thus made regardingwhen a wheel is likely to skid. The system is of course also able todetect that a wheel is skidding. If a skid is predicted or detected, thesystem reacts by adapting the braking effort in view of the input data,which includes vertical loads or changes in vertical loads, wheel speed,friction coefficient μ, slip λ, aircraft speed, and brake pressure. Thesystem can then seek to avoid such skids by regulating the brakingtorque applied accordingly.

When a skid is detected on a wheel the braking is reduced so that it isallowed to spin up. Once the wheel has spun up satisfactorily thebraking actuation is reapplied. The way in which the braking is reducedis determined by all, or some, inputs received by the control system,including vertical loads or changes in vertical loads.

The monitoring of the vertical loads or changes in vertical loads allowsthe system to estimate the μ-slip characteristics of the tyre/groundinterface. This information is then used to predict skids before theyoccur. μ can be calculated/estimated using equation (1) (mentioned abovewith reference to FIG. 2); the other parameters in the equation will beknown (R, I, T_(R)), or can be ascertained from measurements (dω/dt,F_(z), T_(B)).

Thus, the control system infers the underlying interactions between tyreand ground, and learns from said information to improve the control andavoidance of skids.

The increased intelligence of the system by means of monitoring verticalload, or changes in vertical load, increases efficiency in braking andcan reduce, if not eliminate, the occurrence of skids, thus reducingtyre wear.

The tyre friction force, due to braking, is a combination of thevertical load F_(z) and the friction coefficient μ. The system monitorsF_(Z) and is thus able to measure when the frictional force is likely tobe reducing. Consider, for example, a situation where the anti-skidsystem detects a reduction in the vertical load acting on thetyre/ground interface. If the tyre/runway interface is near the optimumfriction coefficient there is a chance that a reduction in vertical loadcould lead to a skid. The system attempts to anticipate the skid andreduces the braking torque before the skid occurs. Conversely, thesystem increases the braking torque, if the vertical load increases.

If the vertical load is known, it is possible to estimate the frictioncoefficient for the tyre/ground interface. The rotational moment ofinertia—I, and Rolling radius—R are known in advance, remainsubstantially constant and are pre-programmed into memory accessible bythe anti-skid controller 1. The torque due to rolling resistance—T_(R)may be estimated in view of the values of other measured parameters. Thetorque due to rolling resistance, T_(R), may be sufficiently small thatit can be assumed to be zero in certain cases. The system calculates thewheel angular acceleration—dω/dt from the wheel speed sensor andcalculates the braking torque T_(B) from the brake pressure P and apre-determined torque/pressure relationship for the brake. Bycalculating the friction coefficient μ and slip λ the system determinesinformation concerning a μ/slip curve, which is then used to optimisethe braking performance.

The load variation can be very high, particularly during, and soon afterlanding, therefore, vertical load measurement will be particularlybeneficial in an aircraft based environment.

FIG. 4 shows a flow diagram illustrating the method of operation of thebraking control apparatus shown in FIG. 1. The parameters used in themethod include known parameters (indicated by box 10) and measuredparameters (indicated by box 11).

The known parameters 10 include the moment of inertia I of the rotatingwheel system including the wheel, tyre and brake (box 10 a), the torqueT_(R) due to rolling resistance (box 10 b) and the rolling radius R ofthe tyre (box 10 c). The measured parameters 11 include the verticalload F_(Z), or a parameter relating to the vertical load, (box 11 a),the pressure P in the hydraulic system supplying the brake (box 11 b),the temperature T of the brake (box 11 c), the angular velocity ω of thewheel (box 11 d), and the speed V of the aircraft along the ground (box11 e). Whilst it is difficult reliably to measure directly the brakingtorque T_(B) applied, such a measurement may also be made, whichpossibility is represented by box 11 f.

The parameters 11 are measured periodically and then used by theprocessor in various calculations. The initial calculating steps areindicated in FIG. 4 by boxes 12 a to 12 e.

In a case where the vertical load F_(Z) between the wheel and ground isnot directly measured, the processor estimates the vertical load fromthe measured parameter that relates to the vertical load F_(z). Such astep may require multiplying the measured parameter by a scaling factorand then offsetting that value by a pre-set amount. Depending on theparameter measured, other calculations may be required in order toestimate the vertical load (for example, if the relationship between themeasured parameter and the vertical load is non-linear). Estimating thevertical load is indicated by box 12 a in FIG. 4.

Boxes 12 b and 12 c illustrate the method of estimating the brake torque(assuming that the brake torque has not been measured directly in thestep represented by box 11 f). Box 12 b represents a step of estimatingthe brake gain, that is the scaling factor that defines the relationshipbetween the brake torque applied in response to a given hydraulicpressure in the brake system and that hydraulic pressure. Variousfactors affect the brake gain, including the brake temperature T, andthe angular velocity ω of the wheel. The gain can be estimated by usinglook-up tables in which a value of the gain is given in relation to arange of values of brake temperature, brake pressure and angularvelocity ω of the wheel. Intermediate values may be calculated byinterpolation. The brake torque is then estimated (see box 12 c) bycalculating the product of the brake gain ascertained in the previousstep and the brake pressure P.

The initial estimate of the brake gain. (see box 12 b) may be improvedby means of iterative/extrapolation techniques. Once a relationshipbetween the braking torque and the brake pressure has been estimated,the accuracy of that estimate can be assessed by making furthermeasurements of various parameters thereafter, predicting what themeasurements will be and comparing the measurements made with thosepredicted. An estimate may then be made of the error in the brake torqueas originally calculated compared to the brake torque that, it iscalculated, would have to have been applied in view of the values of thefurther measurements. Mathematical techniques enabling two unknowns froman equation to be ascertained from sets of measured sample data of theother parameters of the equation are advantageously used to estimatevalues of the two unknown parameters in this case (i.e. the brakingtorque and the friction coefficient μ). Such mathematical techniques mayfor example use the initial estimated values of the unknowns as astarting point and may thereafter produce improved values for theunknowns. Such techniques may for example be iterative in nature. Theresulting values calculated by such techniques may then be used toimprove the model/equation with which the value of the brake gain iscalculated.

Box 12 d represents a calculation in which the amount of slip betweenthe tyre and ground is calculated by the formula

$\lambda = {1 - {\frac{\omega\; R}{V}.}}$

The friction coefficient μ is estimated (see box 12 e) by means of thefollowing calculation:

$\begin{matrix}{\mu = \frac{{I\frac{\mathbb{d}\omega}{\mathbb{d}t}} + T_{B} + T_{R}}{F_{z}R}} & (3)\end{matrix}$

The estimates of μ and λ (represented by boxes 12 d and 12 e) arerecorded over time. The record of the relationship between μ and λ isused (see box 13 a) to determine the regions of unstable and stablebraking. As described above in relation to FIG. 3, the condition atwhich unstable braking occurs is defined by λ>λ_(μmax). The values of μand λ are therefore monitored to assess whether or not conditions areapproaching those at which unstable braking occurs.

Box 13 b represents the main skid prediction/detection algorithm. Thecalculated values of μ, λ, the vertical load F_(Z) and the measuredparameter of the brake pressure P are all monitored to assess both whenthe aircraft wheel has started skidding and also to predict when skidsmight occur. If a skid is predicted or detected the braking pressure ischanged accordingly. There are various means by which a skid may bedetected. The algorithm will deem that a skid has been detected ifeither the calculated slip λ exceeds the slip at which the frictioncoefficient μ is at a maximum (i.e. if λ>λ_(μMAX)) or the rate of changeof slip exceeds a preset threshold,

$S_{\max}\mspace{14mu}{\left( {{{i.e.\mspace{14mu}{if}}\mspace{14mu}\frac{\mathbb{d}\lambda}{\mathbb{d}t}} > S_{\max}} \right).}$There are also various ways in which whether a skid will occur in thenear future may be predicted. The algorithm assesses either whether theslip λ is approaching the slip at μ_(max) (i.e. if λ continues toincrease in the same way as it has previously, will it exceed λ_(μmax)within a given period of time), or whether the vertical load decreasesbelow a preset threshold (or the rate of change of vertical loaddecreases below a preset negative threshold) and the slip is greaterthan a preset value of slip, which is less than but close to λ_(μmax).

If a skid is detected or is predicted then the skid protection/detectionalgorithm reduces the brake pressure applied at a rate dependent on allof the factors consisting of the difference between the calculated slipλ and λ_(μmax), the rate of change of slip dλ/dt, the measured brakepressure P, the measured vertical load F_(Z), and the rate of change ofvertical load dF_(Z)/dt.

The algorithm continuously monitors for the detection and prediction ofskids and will, where appropriate take action, for example by causingthe brake pressure to be changed appropriately (the application of thebrakes being represented by box 14). When a pilot of the aircraftdemands a level of braking to be applied, the appropriate braking torquewill be applied, unless it is determined by the algorithm that therequested braking torque is too high. Thus, the braking algorithmeffectively causes an optimum braking torque to be applied, the torqueapplied not being greater than a maximum torque equivalent to the levelof braking demanded by the pilot. If conditions are such that thealgorithm determines that the braking torque demanded by the pilot istoo high, a lower torque will be applied; if conditions then change sothat the algorithm judges that a higher braking torque may be safelyapplied, the braking torque applied will increase (up to a maximumtorque equal to the braking torque demanded by the pilot).

Various modifications may of course be made to the above-describedembodiment without departing from the spirit of the present invention.For example, alternative means of measuring/inferring the vertical loadsor changes in the vertical loads may be used instead of, or in additionto, the strain gauges. Examples of such alternative means are describedbelow.

Calculations of vertical load can be performed in respect ofmeasurements of tyre pressures. Accelerometers, such as pitch, rolland/or yaw monitoring devices, and/or x, y and z axis accelerationmonitoring devices can be used to infer vertical loads or changes invertical loads on one, or a number of, wheel(s). Oleomatic pressuresensors in the oleo shock absorbers of the landing gear may also providean indication of the vertical loads sustained.

As mentioned above, rather than estimating the braking torque T_(B)applied, the torque could alternatively be measured directly. Torquesensors could for example be provided to measure the torque directly foreach wheel concerned.

1. A method of applying a braking force to a wheel of an aircraft movingalong the ground, wherein the method comprises the steps of: estimatingthe conditions at which the wheel would skid; and applying the brakingforce to the wheel in dependence on the results of the estimating step;wherein the estimating step including both taking into account thevertical load transmitted between the ground and the wheel and takinginto account a variable relating to the braking force to be applied. 2.A method according to claim 1, wherein the braking force is applied at alevel ax which it is judged that the conditions for skidding will not bemet whilst maintaining effective braking.
 3. A method according to claim1, wherein the estimating step includes taking into account a variablerelating to time, whereby estimating the conditions at which the wheelwould skid includes estimating when the wheel is likely to skid.
 4. Amethod according to claim 1, wherein the estimating step includes theperformance of a calculation, in which a parameter relating to thevertical load transmitted between the ground and the wheel is taken intoaccount.
 5. A method according to claim 4, wherein a slip parameter istaken into account when performing the calculation, the slip parameterbeing such that the amount of slip between the ground and the wheel andthe slip parameter are interrelated.
 6. A method according to claim 5,wherein data is ascertained regarding the relationship between slip andthe ground to wheel friction coefficient and at least some of the dataso ascertained is used in the calculation.
 7. A method according toclaim 5, wherein the method includes recording, over time, data relatingto the relationship between the value of the friction coefficient andthe value of slip.
 8. A method according to claim 5, wherein the methodincludes ascertaining the slip parameter relating to the slip betweenthe ground and the wheel by means of measuring parameters relating tothe aircraft speed and the speed of the periphery of the wheel.
 9. Amethod according to claim 1, wherein the method further comprises a stepin which a prediction is made regarding how the vertical load willchange and the prediction is taken into account when performing theestimating step.
 10. A method according to claim 1, wherein the methodis so performed that, if a skid is detected, the braking force isreduced in a way that takes into account data relating to the verticalload transmitted between the ground and the wheel.
 11. A methodaccording to claim 1, wherein braking is applied by means of a hydraulicsystem, and the method includes a step of ascertaining a parameterrepresentative of the hydraulic pressure in the brake system, the methodincluding a step of calculating the braking force to be applied to thewheel, the parameter being taken into account when performing thatcalculation.
 12. A method of applying a braking force to a wheel of anaircraft moving along the ground, wherein the method comprises the stepsof: ascertaining a first parameter dependent on the amount of slipbetween the ground and the wheel; ascertaining a second, parameterdependent on the ground to wheel friction coefficient; ascertaining athird parameter dependent on the vertical load transmitted between theground and the wheel; recording, over time, data relating to therelationship between the first and second parameters; estimating theconditions at which the wheel would skid, the estimating step includingthe performance of a calculation, in which the first, second and thirdparameters are taken into account; and applying a braking force to thewheel in dependence on the results of the estimating step.
 13. A methodaccording to claim 12, wherein a control unit controls the braking forceapplied such that the level of slip nears, but does not exceed, a levelat which unstable braking starts, the control unit using the recordeddata in order to assess the point at which unstable braking starts. 14.A method according to claim 12, wherein at least some of the datarelating to the relationship between the first and second parameters isused in the calculation preformed in the estimating step.
 15. A methodof applying a braking force to a wheel of an aircraft moving along theground, wherein the method comprises the steps of: making a predictionconcerning how the vertical load transmitted between the ground and thewheel will change; estimating the conditions at which the wheel wouldskid, the estimating step taking into account the prediction concerninghow the vertical load will change; and applying a braking force to thewheel in dependence on the results of the estimating step.
 16. A methodof applying a braking force to a wheel of an aircraft moving along theground, the brakes being actuated by means of a hydraulic system,wherein the method comprises the steps of: estimating the conditions atwhich the wheel would skid; the estimating step taking into account thevertical load transmitted between the ground and the wheel; ascertaininga hydraulic pressure parameter representative of the hydraulic pressurein the brake system, calculating the braking force to be applied to thewheel taking into account the results of the estimating step and thehydraulic pressure parameter; and applying the braking force socalculated to the wheel.
 17. A method according to claim 16, wherein themethod includes a step of estimating how the braking force appliedchanges with changes in other variables and varying the braking pressureapplied to account for the changes in such other variables.
 18. Abraking control apparatus for controlling the braking of an aircraftwheel and a processor associated wit the braking control apparatus,wherein: the apparatus is connectable to the brakes of at least onewheel of an aircraft, the processor is able to be connected to receivein use signals relating to the vertical load transmitted between theground and the aircraft wheels; the processor is so arranged that in useit performs a calculation using data derived from the signals receivedby the control apparatus and estimates the conditions at which the wheelwould skid taking into account hot the vertical load transmitted betweenthe ground and the wheel and a variable relating to the braking force tobe applied; and the control apparatus is so arranged that in use thecontrol apparatus actuates the brakes in dependence on the results ofthe calculation performed by the processor, whereby the controlapparatus is able to control the actuation of the brakes taking intoaccount the vertical load and other conditions that affect thelikelihood of skidding.
 19. An aircraft comprising a braking controlapparatus and processor according to claim
 18. 20. A braking controlapparatus for controlling the braking of an aircraft wheel and aprocessor associated with the braking control apparatus, wherein: theapparatus is connectable to the brakes of at least one wheel of anaircraft; the processor is able to be connected to receive in usesignals relating to the vertical load transmitted between the ground andthe aircraft wheels; the processor is so arranged that in use itperforms a calculation using data derived from the signals received bythe control apparatus, the calculation including: making a predictionconcerning how the vertical load transmitted between the ground and thewheel will change, and estimating the conditions at which the wheelwould skid taking into account both the vertical load transmittedbetween the ground and the wheel and the prediction concerning how thevertical load will change, and the control apparatus is so arranged thatin use the control apparatus actuates the brakes in dependence on theresults of the calculation performed by the processor, whereby thecontrol apparatus is able to control the actuation of the brakes takinginto account the vertical load and other conditions that affect thelikelihood of skidding.
 21. An aircraft comprising a braking controlapparatus and processor according to claim
 20. 22. A control unit and alanding gear assembly for an aircraft, the assembly including at leastone aircraft wheel, the control unit being able in use to actuate thebrakes of said at least one wheel, wherein: the control unit includes aprocessor, which is connected to receive data signals relating to thevertical load transmitted between the ground and the aircraft wheels,and which in use performs a calculation using data derived from the datasignals received by the processor and estimates the conditions at whichthe wheel would skid, the estimating step taking into account both thevertical load transmitted between the ground and the wheel and avariable relating to the braking force to be applied, and the controlunit is so arranged that in use the control unit actuates the brakes independence on the results of the calculation performed by the processor.23. An aircraft comprising a control unit and a landing gear assemblyaccording to claim
 22. 24. A control unit and a landing gear assemblyfor an aircraft, the assembly including at least one aircraft wheel, thecontrol unit being able in use to actuate the brakes of said at leastone wheel, wherein: the control unit includes a processor, which isconnected to receive data signals relating to the vertical loadtransmitted between the ground and the aircraft wheels, and which in useperforms a calculation using data derived from the data signals receivedby the processor and estimates the conditions at which the wheel wouldskid, the estimating step taking into account both the vertical loadtransmitted between the ground and the wheel and a prediction concerninghow the vertical load will change, and the control unit is so arrangedthat in use the control unit actuates the brakes in dependence on theresults of the calculation performed by the processor.
 25. An aircraftcomprising a control unit and a landing gear assembly according to claim24.